subject matter; theme; material (for a work)
Refers to the topic or raw material an artist, writer, or researcher draws on for a creative or academic work. Not used for school subjects or grammatical subjects.
The subject matter of this novel is the experience of war.
He paints pictures using everyday events as his material.
This movie is based on a true story.
He wrote a novel based on the theme of war.
He draws his material from everyday life.